1. Field of the Invention
The present invention relates in general to a toroidal type continuously variable transmission, and more particularly to a toroidal power transmission unit employed in the transmission.
2. Description of the Prior Art
In order to clarify the task of the present invention, a known toroidal type continuously variable transmission will be described with reference to FIGS. 9 to 11 of the accompanying drawings. Such transmission is shown in Japanese Utility Model Provisional Publication 63-92859.
For ease of description, the toroidal type continuously variable transmission will be referred to as toroidal type-CVT, hereinafter.
FIGS. 9 and 10 show part of the toroidal type-CVT. In FIG. 9, designated by numerals 1 and 2 are input and output cone discs which are arranged on a common rotation axis "O1", and designated by numerals 3 and 3 are power rollers which are each operatively interposed between the input and output cone discs 1 and 2. The power rollers 3 and 3 are arranged to face each other with the axis "O1" placed therebetween. The input and output cone discs 1 and 2 and the two power rollers 3 and 3 constitute one toroidal power transmission unit.
In a double cavity toroidal type-CVT, two, that is, front and rear toroidal power transmission units are employed, which are coaxially arranged on the axis "O1" with their output cone discs 2 and 2 connected in a back-to-back connecting manner.
The power rollers 3 and 3 of the front transmission unit are rotatably supported by respective trunnions 4 and 4 as is seen from FIG. 9, and the power rollers of the rear transmission unit are rotatably supported by respective trunnions 5 and 5 as will be understood from FIG. 10.
As is understood from FIG. 9, the trunnions 4 and 4 (or, 5 and 5) have respective upper ends connected through an upper link 6 and respective lower ends connected through a lower link 7. The connection of these parts is so made that each trunnion 4 (or 5) can pivot between a neutral position as shown in FIG. 9 wherein a rotation axis "O2" of the power roller 3 intersects the rotation axis "O1" of the input and output cone discs 1 and 2 and an offset position wherein the trunnion 4 (or 5) is inclined toward a pivot axis "O3" of the power roller 3 which intersects the rotation axis "O2" at right angles. The trunnion 4 (or 5) pivots about the pivot axis "O3".
For achieving the above-mentioned offset movement, each trunnion 4 (or 5) has a shaft 10 connected thereto through a connecting pin 12, the shaft 10 extending in the direction of the pivot axis "O3".
A piston 14 is connected to the shaft 10. The piston 14 is formed with a hollow boss portion 14a which receives therein the shaft 10. Due to movement of the piston 14 along an axis of the hollow boss portion 14a, the above-mentioned pivotal movement of the trunnion 4 (or 5) is induced.
As is seen from FIG. 9, the hollow boss portion 14a of the piston 14 of the front transmission unit, which is incorporated with the left trunnion 4, passes through both a cylinder body 15 which houses therein the piston 14 and a control valve body 18 which produces a hydraulic pressure for actuating the piston 14, and the hollow boss portion 14a of the piston, which is incorporated with the right trunnion 4, passes through only the cylinder body 15. While, as is seen from FIG. 10, the hollow boss portions 14a of the pistons 14 of the rear transmission unit, which are incorporated with the trunnions 5, pass through the cylinder body 15.
The control valve body 18 is equipped with a speed control valve 17. That is, upon receiving a gear ratio command, the speed control valve 17 moves the pistons 14 with a hydraulic pressure corresponding to the content of the command. With this, each piston 14 pivots through the corresponding shaft 10 the trunnion 4 (or 5) between the above-mentioned neutral position of FIG. 9 and the above-mentioned offset position. Upon this, each power roller 3 is pivoted about the pivot axis "O3" while receiving a component force from the input and output cone discs 1 and 2, so that the rotation speed of the output cone disc 2 is continuously varied relative to that of the input cone disc 1. That is, due to continuously varying contact points of the power rollers 3 to the input and output cone discs 1 and 2, the gear ratio is continuously varied between the input and output cone discs 1 and 2. In other words, a continuously variable speed change is obtained.
As is seen from FIGS. 9 and 10, the shaft 10 of one of the trunnions 4 of the front transmission unit has a lower end projected downward from the control valve body 18, to which a precess cam 9 is secured. A speed change link 21 is incorporated with the precess cam 9, through which the above-mentioned movement of the trunnion 4 is fed back to the speed control valve 17. Due to this feed back control, the trunnions 4 and 5 of the front and rear transmission units are forced to return toward their original positions, and when the existing gear ratio becomes in agreement with the target value of the command, the trunnions 4 and 5 are returned to the original positions bringing back the power rollers 3 to their neutral positions. With this, the gear ratio is kept at the target value.
However, due to inherent construction, the above-mentioned toroidal type-CVT tends to have the following drawback.
That is, under operation of the toroidal power transmission unit, each power roller 3 is hardly compressed between the input and output cone discs 1 and 2 with a force corresponding to a delivered torque therebetween because the torque delivery has to be made by shearing oil film placed between the power roller 3 and each of the input and output cone discs 1 and 2. Accordingly, as is understood from FIG. 11, under operation, each power roller 3 is applied with a marked thrust "F" in a direction to be driven out from the input and output cone discs 1 and 2. Thus, for suppressing undesired displacement of the trunnion 4 (or 5) even when such marked thrust "F" is applied thereto, the upper ends and lower ends of the paired trunnions 4 and 4 (or, 5 and 5) are connected through the respective upper and lower links 6 and 7, as is described hereinabove.
Accordingly, as is shown by phantom lines in FIG. 11, under operation of the transmission, by the marked thrust "F", the trunnion 4 (or 5) is resiliently flexed or bent slightly but in a certain degree with the upper and lower links 6 and 7 serving as fulcrums, so that the shaft 10 and the piston 14 are forced to incline, as shown. However, this inclination tends to generate a marked friction force between the hollow boss portion 14a of the piston 14 and a wall of a bore the cylinder body 15 through which the boss portion 14a passes. Of course, to achieve smoothed and reliable operation of the toroidal-type CVT, elimination of such friction force is needed.